Waste paper materials often contain color which must be removed in order to make decolorized secondary pulp for the production of high brightness recycled paper products. Color is caused by dyes, pigments, inks, and other color-causing compounds added in the production and printing of final paper products. Color also may be caused by residual lignin which is present in secondary pulps prepared from waste paper materials containing unbleached paper such as newsprint, cardboard, unbleached kraft, and the like. Color is usually removed or stripped from secondary pulps by treatment with chlorine or chlorine-based bleaching agents such as sodium hypochlorite, hypochlorous acid, and chlorine dioxide. Because of developing trends toward reducing or eliminating chlorine-based bleaching processes due to environmental regulations and consumer preferences, new color removal or color stripping processes which reduce or eliminate chlorine-based bleaching agents are needed in the secondary fiber recovery industry.
Oxygen delignification of pulp from corrugated containers is described in an article by A. de Ruvo et al entitled "Upgrading of Pulp from Corrugated Containers by Oxygen Delignification" in Tappi Journal, June 1986, pp. 100-103. Contacting of pulp with oxygen and alkali at elevated temperatures removes lignin, as measured by a reduction in kappa number, and also produces paper with improved tensile strength and burst index.
U.S. Pat. No. 4,416,727 discloses a process for recovering and maintaining the brightness of fibers from wet-strength resin-coated waste paper furnish by pulping the furnish in an alkaline solution and contacting the pulp with oxygen to dissolve the polymeric wet-strength resins followed by recovery of the acceptable fibers in a washing step.
German Patent Application DD 247 934 A1 discloses a process for improving the optical homogeneity of deinked waste paper materials by the addition of oxygen and alkali at elevated temperatures. The fiber material thus treated is available for further treating, preferably for papers for graphic purposes.
L. D. Markham and C. E. Courchene, in an article entitled "Oxygen Bleaching of Secondary Fiber Grades" published in Tappi Journal, December 1988 at pp. 168-174, disclose oxygen bleaching for the removal of dirt and for improving the brightness and bleachability of recycled pulps. Oxygen delignification of a number of mixed waste paper materials as determined by kappa number and brightness measurements were carried out in laboratory and pilot plant tests. The use of oxygen-hypochlorite and oxygen-peroxide sequences were studied briefly. The authors conclude that oxygen bleaching with appropriate screening and cleaning technology can produce bleached or unbleached pulp of high quality from a low-grade wastepaper furnish.
In a paper given at the Tappi Pulping Conference 1990, Oct. 14-17, Toronto, entitled "Environmentally Safe Bleaching of Post Consumer Waste Papers", J. E. Angulo describes several chlorine-based and non-chlorine-based bleaching sequences for secondary fibers. Several sequences using oxidative reinforced extraction stages, such as Ep (extraction stage with peroxide) and Epo (extraction stage with peroxide and oxygen), were disclosed for the reduction of chlorine-based chemical doses in combination bleaching sequences. In addition, data for the sequences Epo-Z-P, Epo-P, Z-Epo-P, and Epo-P-Y were disclosed (Z denotes ozone and Y denotes hydrosulfite).
Non-chlorine, non-oxygen bleaching sequences for secondary fibers have been described in four other recent papers. T. Joachimides and M. Hache in 1990 Pulping Conference, TAPPI Proceedings, pp. 507-513, in an article entitled "Bleaching Deinked Pulps" discuss the use of sodium hydrosulfite, formamidine sulfinic acid, and hydrogen peroxide for bleaching deinked newsprint. M. T. Berger et al disclose the use of hydrogen peroxide and/or sodium hydrosulfite for bleaching secondary fibers in deinked and colored secondary pulp in an article entitled "Advanced Bleaching Technology for Secondary Fibers" published in the proceedings of the Pacific Paper Expo, MB, Howe Sound, Canada, Fall 1990. W. H. Matzke and H. H. Selder discuss the use of hydrogen peroxide and/or sodium hydrosulfite for bleaching secondary fibers from low-brightness waste paper stock in a paper entitled "Various Approaches for Understanding and Improving Secondary Fiber Brightness" in Recycling Paper: From Fiber to Finished Product, pp. 698-708, TAPPI Press, 1990. In a paper entitled "Bleaching with Reductive Agents" given at the North Carolina Chlorine-free Bleaching Workshop, Mar. 18-20 1991, N. Liebergott discloses conditions and results for the bleaching of colored secondary fiber pulp using various sequences including ozone, hydrogen peroxide, sodium hydrosulfite, and formamidine sulfinic acid.
Environmental regulations and consumer demands will restrict and reduce the future use of chlorine-based bleaching agents. In addition, the growing incentives for waste paper recycling will require greater use of lower-quality grades of waste paper which contain higher amounts of colored paper products, and this trend will require more economical and efficient methods of color removal in the secondary fiber recovery industry. The present invention addresses these needs by describing new and useful methods for color removal from secondary pulps used for the manufacture of recycled paper products.